Mechanisms responsible for the striking stability with which low copy number plasmids are inherited continue to be the subject of our research. In the period of this report, considerable understanding has been gained of the structure of ori and parS, the cis-acting sites involved respectively in plasmid replication and plasmid partition. The emphasis this year has been on how these sites change in shape and activity as a consequence of interactions with specific plasmid-determined and host-determined proteins or by interaction with regulatory DNA sequences. The development of means for recognizing and measuring structural changes that occur when the P1 initiator protein, RepA, interacts with its binding sites will enable the specification of RepA domains implicated in each of its several functions. Of particular significance is the genetic evidence suggesting that three heat shock proteins participate in concert to increase the affinity of RepA for its binding sites. At least one of these proteins, DnaK, is highly homologous to proteins of higher organisms. The most dramatic of the interactions studied, because of the unusual magnitude of its effect, is the binding of the accessory host factor IHF to parS. This binding induces a 10,000 fold increase in the affinity of the P1-encoded partition protein ParB for the plasmid centromere, parS, provided that the plasmid DNA is supercoiled.